1. Field of the Invention
The present invention relates to a static type memory device and, more particularly, to a static type memory device employing MES type field effect transistors (MESFETs) formed of compound semiconductor material such as GaAs.
2. Description of the Related Art
Static memories (SRAMs) have been widely utilized in various fields because of their high-speed operations. Recently, many approaches have been made on developments of SRAMs employing GaAs-MESFETs to further increase operation speed. In a conventional SRAM employing MESFETs, each of memory cells includes a flip-flop supplied with a high-side power source V.sub.DD and a low-side power source V.sub.SS and a pair of transfer MESFETs connected between the flip-flop and a pair of digit lines. A plurality of pairs of digit lines are connected to a pair of common data lines through a plurality of pairs of column selection MESFETs. A read-out circuit and a write-in circuit are connected to the pair of common data lines. The write-in circuit is supplied with the high-side and low-side power source V.sub.DD and V.sub.SS and operatively applies true and complementary input data signals having amplitude between V.sub.DD and V.sub.SS at maximum to the pair of common data lines. The input data signals applied to the pair of common data lines are transferred to one pair of digit lines through a selected pair of column selection MESFETs thereby to set a state of the selected memory cell. In general, a ground potential (0 V) and -1.9 V employed as V.sub.DD and V.sub.SS in the conventional SRAM. The flip-flop of the memory cell assumes -1.3 V and -1.9 V at a pair of input/output nodes thereof, because the Schottky diodes between a gate and a drain of the high-level output side MESFET is forward biased, and because the Schottkey diode of the gate and a drain has a forward voltage of approximately 0.6 V. In such memory device, the selected pair of digit lines are supplied with input data signals of 0 V and -1.9 V by the write-in circuit. Therefore, when the input data of -1.9 V is applied to the high level output side MESFET. The voltage difference therebetween becomes 0.6 V. For write-in operation, the flip-flop must be turned by the 0.6 V.
For turning the flip-flop, parastic capacitors of the MESFETs in the flip-flop must be first charged prior to drive MESFETs. However, because the MESFET's have large sizes compared to the column selection MESFET's, it takes a very long time until the parasitic capacitors are charged-up by the above-mentioned small voltage 0.6 V. As a result, a speed of the memory is not sufficiently improved particularly in a write operation in the conventional SRAM.